The only time I was faced with injecting any of mine (dogs not cats) was when a litter went down with a gastric bug and rather than take them too and from my vet (a distance away), he gave me what was needed to inject them. Hated doing it. Ditto with another situation where another vet, in another town, gave me two lots of oxytocin to inject my b itch who was in secondary inertia. I got one done, but hated doing it so I took her back to that vet who by then, as there'd been no improvement, went on to do a CSection.
This is something student vets have to learn how to do, to get it right. I suggest you get back to your vet for help with this!!